A vehicle-mounted air conditioner in an electric vehicle or a fuel cell vehicle with no engine has a compressor including an electric motor as a power source for compressing and circulating a refrigerant. Since the electric motor needs to be rotated at a desired rotational speed according to a command from a main control apparatus of the air conditioning apparatus, a separate control apparatus is required. This control apparatus includes an electrical circuit or an electronic circuit. Specifically, the control apparatus includes electronic elements such as a central processing unit and a memory, and also a switching element (power transistor element) such as an IGBT (Insulated Gate Bipolar Transistor) or an FET (Field Effect Transistor) for constituting a so-called inverter circuit (switching circuit). There is an electric compressor in which the control apparatus, a compression mechanism, and an electric motor are incorporated into one casing for saving space. Hereinafter, this electric compressor is sometimes referred to as an integrated electric compressor.
The switching element has a function of supplying large electric power to the electric motor and controlling the rotational speed of the electric motor. However, the control by the control apparatus is to control the electric motor to rotate at a desired rotational speed according to the command from the main control apparatus. During the control, power consumption becomes too high, and an overcurrent sometimes flows that may damage a high voltage component (hereinafter sometimes simply referred to as component) mounted in the control apparatus of the integrated electric compressor. Thus, the switching element and other components are increased in temperature and damaged (heat damage). In an extreme case, it is supposed that the components rupture or catch fire. The heat damage may occur to components that constitute the electric motor.
Patent Documents 1 and 2 propose protecting a control apparatus of an integrated electric compressor from heat damage.
Patent Document 1 proposes an electric compressor integratedly including a compression mechanism that sucks in and compresses a refrigerant, an electric motor that drives the compression mechanism, and an electrical circuit that controls a rotational speed of the electric motor, wherein the compressor includes protection control means for increasing a rotational speed of the compression mechanism, that is, a rotational speed of the electric motor when a temperature of the electrical circuit exceeds a predetermined temperature, and further when an actual rotational speed of the compression mechanism is a predetermined rotational speed or lower.
This proposal is based on the following findings. Specifically, when the rotational speed of the compression mechanism is relatively low, a cooling effect by the refrigerant falls below calorific values of the electrical circuit and the electric motor, and thus the cooling effect by the refrigerant increases with increasing rotational speed. On the other hand, when the rotational speed of the compression mechanism is relatively high, the calorific values of the electrical circuit and the electric motor exceed the cooling effect by the refrigerant, and thus the temperatures of the electrical circuit and the electric motor compression mechanism increase with increasing rotational speed. Thus, in Patent Document 1, when the temperature of the electrical circuit exceeds the predetermined temperature, the rotational speed of the compression mechanism is increased to reduce the temperatures of the electric motor and the electrical circuit.
Patent Document 2 is based on the findings that a larger amount of refrigerant to be compressed is circulated at higher rotational speeds of an electric compressor, and in combination with an increasing cooling effect of a switching element by the refrigerant, an electric motor of the electric compressor has a larger current margin in a certain rotational speed region than in other regions (paragraph [0025] in Patent Document 2). Specifically, Patent Document 2 proposes a control apparatus for an electric compressor including a rotational speed limit control portion that maintains a rotational speed of an electric motor within a rotational speed between predetermined upper and lower limit rotational speeds in the case where a certain condition A relating to an inverter output current or a switching element temperature that indicates a state that requires protection of the switching element that constitutes a control apparatus of an integrated electric compressor irrespective of an electric motor rotational speed command from a main control apparatus of an air conditioning apparatus.
In Patent Document 2, for example, it is determined whether an actual operation rotational speed of the electric compressor is lower than a predetermined rotational speed N1 when a maximum value of an inverter output current absolute value during one second becomes higher than 0.9 times a known rated current of the switching element. When the actual operation rotational speed is lower than the rotational speed N1, a current margin is small, and thus the rotational speed is increased by a predetermined rotational speed N3. When the actual operation rotational speed is higher than N1 and also higher than N2, the current margin is also small, and thus the rotational speed is reduced by a predetermined rotational speed N4. Then, it is determined whether a rotational speed control operation needs to be continued.    Patent Document 1: Japanese Patent Laid-Open No. 2004-68807    Patent Document 2: Japanese Patent Laid-Open No. 2007-92636